Surgical implants have been used in a variety of soft tissue repair procedures and include, for example, biologic grafts and meshes used for pelvic floor repair, for hernia repair, for the treatment of stress urinary incontinence, for thoracic wall defect repair, for breast support, and for cosmetic and reconstructive surgery, among others.
Non-absorbable synthetic mesh materials (e.g., polypropylene, etc.) provide durable long-lasting soft tissue repairs. Such materials trigger a biologic foreign body response (FBR) when implanted, establishing chronic inflammation and FBR at the site of implantation. A typical FBR usually starts with a local inflammatory response, accumulation of monocytes and macrophages, followed by fibrinogen deposits, fibrosis and scar tissue formation. This fibrous connective scar tissue becomes integrated into the mesh or forms a capsule around the mesh depending, for example, on the mesh density and pore size. In meshes where the tissue is incorporated into the interstices of the mesh, the fibrous connective tissue forms a strong mechanical link between the mesh and the adjacent tissue. However, the tissue can also force the mesh pore to expand and distort, causing the mesh fabric size to shrink. The extent of the FBR to the non-absorbable mesh generally depends on the amount of synthetic polymer in contact with bodily tissue, with higher density meshes typically triggering a greater FBR than lower density meshes. This greater, chronic FBR that is associated with higher density meshes in turn creates denser, less flexible scar tissue that can correspond to increased procedural complications such as chronic pain. Although a low density, large pore mesh provides for fewer complications and less pain, such a mesh may not have sufficient strength, especially in the first few weeks post implantation and/or may not have adequate handling characteristics for reliable implantation.
Absorbable synthetic mesh materials (e.g., polyglactin) have an initial FBR and scar tissue response that is similar to non-absorbable mesh materials. Such meshes are typically absorbed within six months of implantation leaving behind scar tissue. Interestingly, absorbable meshes do not have good long term outcomes, with evidence suggesting that the scar tissue is not strong enough or does not maintain its strength over time in order to maintain repair.
Biologic graft materials can be subdivided into crosslinked and non-crosslinked materials. Biologic graft materials, like synthetic meshes, trigger inflammation and a FBR. Crosslinking biologic grafts renders the collagen somewhat to enzyme activity and thus resistant to on-going digestion and remodeling of collagen in vivo. Crosslinked biologic graft materials tend to become encapsulated in a fibrous tissue in a manner similar to synthetic graft materials. Non-crosslinked materials exhibit evidence of digestion and remodeling where host collagen is deposited as the graft material is digested.
Biologic grafts materials appear to have lower rates of erosion and dyspareunia than synthetic mesh grafts. However, it is reported that inflammation and FBR associated with biologic grafts (e.g., porcine small intestinal submucosa, fetal bovine dermis, cadaveric human dermis, etc.); especially non-crosslinked biologic grafts, diminish as the graft is degraded and eventually resorbed. As the FBR diminishes, the amount of collagenous tissue in the region diminishes. This is believed to be a key factor for late failures associated with biologic grafts, including grafts used for hernia and pelvic floor repair.
Composite graft-mesh materials have also been developed. Examples include composite implants from American Medical Systems, Inc. (e.g., InteXēn® LP® non-chemically crosslinked porcine dermis attached to a polypropylene mesh) and C.R. Bard, Inc. (e.g., Avaulta Plus® Biosynthetic Support System, employing a porous, acellular sheet of crosslinked collagen attached to a polypropylene mesh). In each case, however, the composite material comprises a porous polypropylene mesh. Porous meshes, however, introduce the possibility that the tissue in-growth will cause the mesh pores to expand and distort, resulting in mesh shrinkage, as noted above.